Nitrogen is widely used in industrial and commercial operations. It is most efficiently and economically produced in large tonnage quantities by cryogenic distillation of air. There has been a continuing effort to improve those processes so as to reduce the energy requirement and the capital cost of the equipment.
When nitrogen is the primary value product from air separation, as opposed to oxygen, the cryogenic production plants and corresponding processes fall into two groupings: single pressure distillation, and dual pressure distillation. The former group is generally lower in capital cost and more compact, and hence tends to be used in smaller capacity plants, whereas the latter (dual pressure) group is more energy efficient, which makes it most economic at larger capacities.
The single pressure distillation category entails feeding at least the bulk of the compressed, cleaned and cooled supply air to a single pressure column, which may or may not be reboiled at the bottom. The bottom liquid is reduced in pressure and placed in latent heat exchange relationship with overhead vapor, thereby being reevaporated and simultaneously providing liquid nitrogen (LN.sub.2) reflux to the column. Product gaseous N.sub.2 is withdrawn from the column overbead. U.S. patents in this category include U.S. Pat. Nos. 3,203,193, 3,217,502, 3,492,828, 3,736,762, 4,400,188, 4,464,188, 4,566,887, 4,594,085, 4,595,405, 4,617,037, 4,662,917, 4,668,260, 4,696,689, and 4,698,079. They differ in regard to bow the column is reboiled, if at all, and in how the necessary refrigeration effect is produced. The '762, '193, '502, '828, '887, '405, '079, '260, and '689 patents disclose no bottom reboil, i.e., the column is simply a rectifier, with the supply air routed to the bottom. The '0188, '828, and '917 patents disclose bottom reboil via recycling N.sub.2 out of the cold box to a compressor, and then back in to the reboiler. The '4188, '085, and '917 patents disclose bottom reboil via total condensation of part of the supply air after compression to a higher-than-column pressure. Finally, the '037 patent discloses bottom reboil via a closed cycle heat pump which circulates air as working fluid.
There are similarly several disclosures of refrigeration method.
Prior art patents which disclose dual pressure distillative production of nitrogen include U.S. Pat. Nos. 4,617,036, 4,604,117, 4,582,518, 4,543,115, 4,453,957, 4,448,595, 4,439,220, 4,222,756 and British Pat. No. 1,215,377. They all involve supplying feed air to a high pressure rectifier, then routing the rectifier bottom product either directly or indirectly to a low pressure distillation column, and several also involve supplying reboil to the low pressure column by latent heat exchange with vapor from the HP rectifier. Most also incorporate a means of increasing the reflux at the top of the LP column, whereby N.sub.2 purity and yield are increased, by exchanging latent heat between LP column overhead vapor and boiling depressurized LP column bottom product.
The '377 patent was one of the earliest disclosures of the basic configuration described above. It included the option of withdrawing some product N.sub.2 from the HP rectifier overhead, in addition to that withdrawn from the LP column overhead. The '957 patent discloses the same basic configuration, with the modifications of a different method of producing refrigeration and elimination of any transport of liquid N.sub.2 from the HP rectifier overhead to the LP column overhead. The '756 patent also involves the same basic configuration, also eliminates flow of LN.sub.2 from HP rectifier overhead to LP column overhead, and discloses yet another variation for producing refrigeration.
The '220 and '595 patents do not involve reboiling the LP column by latent heat exchange between HP rectifier vapor and LP column liquid. Rather, both of those patents disclose refluxing the HP rectifier by exchanging latent heat with boiling depressurized kettle liquid (HP rectifier bottom product). The at 1east partially evaporated kettle liquid is then fed into the LP column for further separation. This same technique has been disclosed in processes for producing low purity oxygen, e.g. U.S. Pat. Nos. 4,410,343 and 4,254,629. The latter patent explains by means of a McCabe-Thiele diagram the advantage of this technique--that feeding 40% O.sub.2 vapor to the LP coIumn is more efficient than feeding 40% O.sub.2 liquid to the same column.
The primary difference between the '220 patent and the '595 patent is that in the '220 patent the LP column is solely a rectifier with no source of reboil other than the vapor feed to it, whereas in the '595 patent the LP column has a stripping section and a reboiler supplied by total condensation of part of the feed air. The latter means of reboiling the LP column is also disclosed in the U.S. Pat. No. 4,410,343 for low purity oxygen producing processes.
The '115 patent discloses a conventional dual pressure configuration with two novelties: the refrigeration is developed by expanding part of the HP rectifier supply air before it is introduced into the HP rectifier; and also part of the supply air is furnished at a pressure intermediate to that of the two distillation columns, and is totally condensed to provide intermediate reboil to the LP column before being fed thereto.
The '518 patent discloses a dual pressure apparatus requiring only a single air supply pressure wherein the lower pressure column is bottom-reboiled by partial condensation of the supply air, which significantly reduces the required supply pressure.
The '117 patent discloses supplying only a minor fraction of the supply air to the HP rectifier, which achieves less than the usual degree of separation, with the remaining air being work-expanded to LP column feed pressure. The resulting N.sub.2 recovery is undesirably low.
The '036 patent does not provide LP column overhead reflux via latent heat exchange with depressurized bottom liquid. Instead, the bottom liquid is evaporated at very close to the bottom pressure, and then is work-expanded. The expansion drives a cold N.sub.2 compressor which increases the delivery pressure of the N.sub.2 product (from the LP column overhead).
In spite of the extensive variety of cryogenic air distillation processes for N.sub.2 production, and the years of search for improvements, problems still remain. Many disclosures seek to increase the efficiency of the distillation column(s), by adding intermediate reboil or intermediate reflux. Unfortunately this has normally required an offsetting undesirable feature, such as lower N.sub.2 recovery, or requiring a stream to be recycled out of and back into the cold box, or not providing any effective means of putting to advantage the refrigeration expander work, or requiring the low pressured column to operate relatively close to ambient pressure (e.g. below 4 atmospheres absolute) where system and line pressure drops become a very significant loss, and also column diameter becomes a significant cost item.
Accordingly it is one object of this invention to provide an improved air distillation process for nitrogen production which overcomes the limitations of the prior art processes by avoiding the above undesirable features. Surprisingly it has now been discovered that a novel combination of elements or techniques previously known in the N.sub.2 -generation art provides the solution to the longstanding problems of increasing the energy efficiency of both the single pressure and dual pressure cryogenic distillation N.sub.2 production processes, while not increasing their cost, by avoiding the above-enumerated disadvantages.